Habsy nyqtjist



Nov. 13, 1928.

A. B. CLARK ET AL ELECTRICAL PICTURE TRANSMITTVING SYSTEMv Filed June 6, 1925 5 Sheets-Sheet 1 ATTORNEY "Q Nov. 13, 1928. LSQLM? -4 A. B. CLARK Er AL.

ELECTRICAL PICTURE I TRANSMI'TING SYSTEM Filed Junee. 1925 .Y s sham-,sheet 2 7 60 I L E Ms; www

5l INVENTORS ATTORNEY m, ma. Y www A. B. CLARK El' AL ELECTRICAL PICTURE TRANSMITTING SYSTEM Filed June 6, 1925 5 sheets-sheet 3 dez/ce l Lr Lier demce ATTONEY `"rara Aflfriace.

AL'VA B. CLARK, OF BROOKLYN, NEW vYORK; HARRY NYQUIST, OF MILLBURN, NEW JERSEY; AND DAITFORTH K. GANNETT, GF EACKSON HEIGHTS, 'NEW YORK, .AS- SIGNORS 'IO AMERICAN TELEFHONEAND TELEGRAPH COMPANY, A CORPORATION y OFKNEW YORK.

' ELECTBCAL PICTURE-TRANSMTTING SYSTFJM.

Application filed June 6,

An object of our .invention is to provide a new and improved system for electro-optically transmitting images o1"- pictures and other objects which is adapted to give a well defined image or picture at the receiving` end correspondingclosely in its tone values to the picture or'object from which transmission is made at the sending end.` Another object of our invention is to'provide such a system with the transmitted current controlled as such a function ot the light or' the successive elements ofthe picture or object, that incidental changes in the transmission equivalent value ot the line will beef negli-` gible eiiect. Still another object oi our invention is to provide against any blurring ot the received image or picture due to transmission ot' the diilerent component frequencies ot the-current at diilerent rates of speed. These and various other objects of `our invention willlbecome apparent on consideration ofl a limited number ot specific examples Y. of practice in accordance with the principle ofthe invention which we have chosen to disclose in the :following specification, taken with the accompanying drawings. It will be understood that the following description relatesvmore particularly to these examples of the practice of the invention and that the invention will Vbe dened in the.. appended claims. y

Referring to the drawings, Figure l is -a curve sheet `showing a line rcurrent as a function oit' picture `density-in vseveral diiierent relations that will be discussed hereinafter; Fig. 2 is a symbolic diagrainwof a transmitting system embodying oursinvention in a certain form; FigB is a detail diagram showing symbols for a vacuum tube valve; Fig. t is a diagram of a modification as compared with Fig. 2; Fig. 5 isa diagram Vfor a transmitting station embodying a further modiiied embodiment of our invention; F 6 is a diagram for the receiving station corresponding to' the transmittino' station of F 5 Fig. 7 is a diagram of a complete system embodying still another modication; Figs. 8 and 9 areA diagrams of certain light orifices disclosed in Fig. V7 andFig. l() is a diagram of a complete system embodying an attenuation equalizer and a phase equalizer. n

ln the transmission of pictures by elecyspot of the nlm ascompared'with 1925. serial N6. 35,323.. v v

tricity it is a well known practiceito cause an electric current togvary in accordance with the light values of .successive elemental areas of the picture to be transmitted; This current or a current corresponding thereto is transmitted from the sending station tothe receiving station and it may be applied at the receiving stationvto control the application of a light to the successive elements of a sensitive surface so as to develop-in that surtaceshades corresponding to the picture from which the transmission is made. f

ln the transmission of such electric currents it'may happen that the transmission lequivalent value ofthe system will change irregularly from time to time. Such changes occur in long distance telephony but are little noticed becauseto the person at the receiving end they are noticed merely as mod'- erate changes in volume of sound. ,But in a picture transmission system such changes are rather serious because they Vcause the received picture to exhibit differences .of shading7 usually in a somewhat streaked manner and quite obviously not naturalrto the picture as it is intended. I

Referring to Figi, this is a logarithmic diagram exhibiting line current in various functional relations to picture density. The scale of picture density in Fig. l is a percentage scale and we define the picture density o a small elemental area ofv the picture expressed in per cent to'be 100 minus the percentage of transmission vof light through that transparency at that spot.

Assume that the picture density for a picture to be transmitted varies over the range from m to n, corresponding to the lightest and ldarkest portionsoip4 the picture respectively. The range i'om m to 'a corresponds to about le TU (transmission units,V delined by the equation NTU=2O leglo fil/z'2 where il and 2 are direct measures of theintensity at the two places compared). InA the case of the linearrelation shown by the curve A, the corresponding variation of the line current will be from g to f. rlhe equation of curve A is 200265. Now, assume that the attenuation on the line is suddenly reduced by the conveniently assumed arbitrary amount 2.9 TU; this will increase the'received line current .Complete y to ma goes only a small fractionofthe way. i`

to 100 per cent picture density. The change at the upper part of the picture densityrange will seem Very pronounced to the eye of an observer, while the'change at the lower part o-ffthatrangewill be much less noticeable,

foittheireason that ythe eye is sensitive to the amount of `light reflected to it from the surface ofthepicture rather than the amount of light absorbed by the picture.

' 'Curve B shows the relation such that the line currentV is .the square of vthevalue of the pictured'ensity; itsequation is z'=O.OOOa-Z2. in this case a range 'ofy density from m to a will correspond -to a range'in line current from 7,; to f, which is twice as great a rang/sof line current as compared with curve A, when efe pressediin 'transmission units. The same change in transmission as was considered for curreA, namely a reduction of attenuation by TU, produces a cha-nge` in the line curr n from `the range 7L, 7 to the range ft f and changes the picture density from the range mf, a Vto mb, uh. It will be'noticed'that in this case the range of picture density is shifted only half. as far as was the case for curye Aq.' Thus,.the use'of the square relation of curve B'instead of the linear relation of curve At for line current as a function of picture density leads tothe result that changes of attenuation .on the line produce only half 'great' changes in picture density. It is ol yious ltlmtyhigher powers than the second power wouldlcause Correspending;v further increase in the' range of line current required and a corresponding,decrease in 'the effect of a. transmission `changeinthe circuit on the densityof the receivedpicture. However, it

lwill be seen'that the change from the curve A to the curve B has not equalized the effect of a'transmission change in the light and dark parts'of the picture, 'since' the ratio of mg, to m is the same as of ub to n.

Y New, consider the cuiVeC, which is an exponential curve drawn to intersect curve fr at the points corresponding to the abscissas i mandayitsequation is z'=0.l31l {1.041}d.

In this case the-same assumed change in trans- Y missiomnamely a decrease of attenuation by 2.9' T U increases the picture density from the range'm, n to the range me, ne. -This is a less changeat the upper. end of the range than before, idecidedly less than 'for curve A, al-

thoughitisfsomewh at lgreater at the lowerl end of the range is much less serious than at the a; its equation is z'=().lr79r` (1.084) d. A" system Y having this relation of line current to picture density .would.requirefas great a power range on the line as required by the curve B. rll

assumed transmission change of 2.5% TU vde-Y crease of attentuation causes only a small change from ato ad, and at the'lower endthe change is from m Jcomm-much lessthanffor the curve C" and nearly asY smallas rer the curve A. The shift at both fthe light-andthe dark ends of the picture densityrange is little' more than half asgmuch for curve D as for curve C; y 'l i Referring to Fi'gjQ', this' illustrates a system for practising "the relationship shownby curve Bin'Fig. l. -Slis a picture transmitting equipment whichzapplies xto the' line eX- tending therefrom anelectromotive v`force' Varying in magnitude according-tothe shade of successive elements of area ofa picture tok be transmitted; that is, the 4darker thepicture Y` element the greater ytheV electroinoti-ve force. The current due to this; electromotiveforce is a modulated carrier current and from the apparatus S1 it goes tothe oppo'sitely connected valves Di and D2.' represented by a simple symbolhere` and elsewhere in the dra-Wings and -as shown Q'in F 3 g with more detail such a Valve is shown in the upper part of'Fig; 3. It consistsof a plate'QQ and a hot cathode 24 in ayacuous con tainer 23. The cathode 24tis inthe formof a filament kept hot by current from the battery 25. This device is a-yalve or rectifier between the terminals 2l andl 26,-V per'mitting current to flowv only in theconyentional direction from 21 to 26. Since the two Valves Dlfand D2 are oppositely connected in Fig. 2,*they permitboth half'waves of current'to pass.

Itis a wellflrnown property of apreperly designed thermionic Valve such asthat shown in Fig. 8, that for imagnitudes of current in the effective direction, the output current Varies approximately as the-squaresf the in-y put electror'notive force. Hence, theelectro-v motive force across `the low resistance r vvaries as the square of the electromotive*iorce'out- Y Each of these valves' is ino v1 ie amplier A andv determines Athe magnitude of Y the current put on the-line L. Theresult is that the current on the line Varies as the-square of the electromotive force from the apparatu S1, that is, the current on the l:line Variesas the square ofthe picture density within-the apparatus S1," whichdetermines the outputy electromotive force therefrom.

'At the receiving end-the linecurrent goes ing end.

to anarnplifierAfand thence through a high resistance'R and across `the two opposite valvesin parallel C, and G2. Since the current through these valves varies as the square of theelectromotive force across them it follows thattheelectroinotive force appliedto the inputof the/amplifier A varies'ajs the square root of the currentfthrough the resist-V ance R; *'l'hus,A the inverse reiationis estab-4 lished at the input of the receiving apparatus S2, so that the current to thev receiving 'apparatus S2 varies linearlywith the'output elec` tromotive foree'fronr .the'se'nding vapparatus S1, although the 'current on the line varies asV sion,

the square of theelectroinotive force at the sending end and the current at S2 atthereceiving end-ofV the line. Thus, the apparatus `of Fig; 2 operates according-to the characteristic B of Fig. land realizes the advantages discussed in connection therewith; l f

Fig. 4 illustrates how a higher power than the squarejma-y be involved. practice.: lt will readily be seein that in Fig. l two tandem sets 'of valves D1, D2 are employed at the send- Thus, the current onthe line correspends to a fourth power instead ofthe square asin"Fig.-2;-1 At the receiving end in Fig. l

two sets ofshunt'valves C1, C2 are employed,

thus giving `two tandem square `root operations, so thattheinput current-for the receivingapparatus Si, varies as the'fourth root of the current onthe line but varies' linearly with the output electromotive force `frornthe sending apparatus Si;

v in Figs. 5 and 6 we have shownhow an "exp'onential relationlsuch as that-of curve C or D may be realiz-eel.' 'Suchfan'eizponential f transparency ofthe film at the spot involved.v

|The transmitted light enters thephotoelectric cell 3% and.determineslthe magnitude or" the current due to the electromotive force of the batter 35. f This. current throuOh the resistance 36 produces a corresponding component of input electroni-otive force to the two-stageE amplifier A, arranged so that its output current is proportional yto film `densityfat 33.-

This output current determines the input voltapplied to the two-way valveDl, and `accordingly .its output current 'varies asl the square of picture' densityfl This current goes through theadjustable-resistance R1" and determines the electromotivey force to another two-stage amplifieriAZ', andso on through"v two -wayl valv'eD 2 and resistance R2, and then a third set of two-stage amplifier A3, twoway'valve D3'and resistance R3. l

It will be seen that multiple taps go tol modul'atorl M through resistances R7', R8, R, and R fromeach stage of the apparatus heretofore described. Each tap by suitable adjust-- ment contributes tomodulator M a `component of inputcorresponding to a respective term of the foregoing convergent' series, and hence the input to modulator M is the desired exponential Vfunction of the picture'density at3'3.zf i At the receiving end, we rely on the eXpanog cycyi ey-152+ where 2 cg/ 0. The quantity dy-1 is obtained inthe iirst amplifier A4 by arrangingthe circuit to add to the voltage of the ampiified'received line lcurrent cy, a portion vof the directy current voltage in the plate. circuit representingunity. The input transformer of this amplifier' must be properly poled to Vgive theY desired algebraic signs to the terms. The remaining amplifiers A5 and AG should each be single-stage and` arranged toi alternate the signs as in the foregoing series. lThe various component-s are assembled through the respective resistances R,1,'R,2,'R13 and R1, and thus aV` current is applied to lightvalve L0 which is a logarithmic function'(that is, an inverse exponential function) of theline current; hence the light valve 'current at the'receiving end is proportional to thephotoelectrie cellV current at the sending end. The lightyvalve 40 determines the Vquantity of light from source 4l yto an elemental area of sensitive ilm`42'which is moved iin synehronism with the sending film 33. The means for accomplishing this synchronism'inay be of well known construction and mode of operation, one preferred method is that disclosed in .British Patent '242.694,

granted .to WesternElectric Con pany,lIn-

corporated,August 8, 1924.

- In vFig. 7 We'v have shown a' modified system which may readily be adapted to give any desired functional relation between the light value in the picture elements andthe current on the line. At rthe sending' end, light `from the source is directedbytlie lens 49 through a small spot ofthe moving film 51. The light transmitted through the film enters the photoelectric cell 52 and determines the current Vtherethroughand accordingly determines the' electromotive force to the amplier corresponding output current from the amplifier y53 goes through the moving conductorsv carrying the'o'scillographmirror 54. Accordingly, this mirror oscillates with an amplitude determined by the magnitude of the current,

llfl

' the path of thelightfto thephoto'electrio :cell

which :in turn. corresponds to fthedegree. of shade in thetspot of thefilm` 51 thatjilies across .The mirror 545y receiveslightfrom: the source' 50. through thelens. 55 and vreflects-this lightkthrough .an opening in .then screens-56 Y (seetalsoFig: 8)'J andfthence througlr a lens to the .fphotoelectriccellpl The light in k:the cell; 57determines the. input. eleotromotive` force for the .amplifier 58; andthe correspond: ing output ycurrent therefrom. goeslto a' modulator 59 and modulates a carrier currentthat.

is puton the'line.- 1

At the receiving end, the received currents go through an amplifier 60 and thence to a light valve 63. -This light valve is a .ribbon of metal carrying the received current in a strong magnetic field so that the ribbon deflects in accordance With the magnitude of the current. Before this -ribbon is an kopening in a screen as shovvn in Fig19, and this 'opening is uncovered-bythe deflection of the ribbon due to the received current. yLight vfrom ythe source' 6l is directed by the lenses 62 vand64 through the light valve 6.3v and on i a sensitive receiving film" 65, Which is moved in synchronismv With .the sendingl film 51. Synchronizingmeans Ina-y be of any. suitable Well knoWn-kiiidsuch' as disclosed in British vPatent 242,694 referred to hereinabove.

" Bygivingthe proper shapey to the opening.

inthe screen 56 atthe sending end,.asshown 1n Fig. 8,'the modulating current can befmade to have any desired single-valued?functional relation to the picturedensity.- The shape shown corresponds to curve (li-Fig. 1. Thus, any one of then-.four functional relations shown-in Fig; lean be obtained and evidently other ksuch ,functional relations would be l equally possibleby. givingthe propershape to the openingin the screen 56.

`Atfthereceivingend,theshape of the opening uncovered' byfthe light valve Willl be 1 such as to. givetheinverse relationias comparedwith" that at the sending end so that the' light tothe sensitive receiving'film will be a. linear function of the picture densityin the sending' film. Figf 9j illustrates suchan` openingy so designed .as to restore linearity when the sending. end isfequipped l'vv/ith. the slot of Fig; 8.

'In the disclosuresfoffY examples. of ourJ invention that havev4 gone' heretofore f lin .this specification it has been taci'tlyassumed that transmission-onithe vline Was distortionless so .that Whatever' transformation uvas, made at the sending endl to makethe cur-renten the linea. non-linear` Yfunction of vpicture density,

.the inverse:.transformation.at the receiving end would restore the. linear relation. In many cases -the. line vvillzintroducey somedistortion,` and `to some yextent Voorrectionrnay be made forthis by adjustmentV at .the receiving end; CFor example, .in Fig. 6 some correction for f distortion on. the line. mayI .be effected. by Y ustm-.ent of the f resistances5 fsuch. as .:RU R5, etc. However, 1in. certain -cases .other measures' may.,beresorted to in. order .to cor.- reet or compensate vforfline distortion.

InFig.. lOwve have shovvn. a symbolicdiagram ofI asystem comprising*picturesending apparatus Sn'whence the current corresponding to picturezdensity, goes vtoa non-'linear devicewhich effects the" desired transforma-YV tion the v current before putting it on .fthe line. This.non-lineandevice may be suchias shown in any one ofFigs. 2, 4,501.17. n l

'Thelinein Fig. flmay.- be assumedntobe aloaded cable circuit Withrepeaters; at proper intervals.` Atthe receiving Yend the current goes first to i an y attenuation equalizer. such fas disclosednin British; yFatent.242,69411 referred to -hereinabove--and then =to a )phase equalizersuch as disclosed .in Patent 'a1-,67,5546Owissued to. H. Nyquist July 3, 19.28. .Thelineawillatdifferent amounts, thus altering=the-.wave

form yfromthat-at the transmitting end." .The

phase .equalizer-Will further Tretard: ltheidif# v ferent components of various` .frequencies byy different 1 amounts, 1 but-sof-that ythe overeall phase retardation is equa-lized; that-fis, .the delay in time oftransmission over the line and through the phase-equal-izerWill be the same for'all frequencies which are of interest. Stated other-Wise the -overf-a'lltphasel shiftfin angular. measure will l. be. proportional dto.. the frequency, l f

Thus, Vthe currentat the receiving'.- end'; after passing through gtheattenuation equalizer and the-phase'equalizenawillzhave thessameftvaveform# as the 1 ont'put from ,1 the. Vnonllinearl -f de@` vice` at thewsending end :This currentavvill" then. go `to an: amplifier '.A, then through?y an inversenon-linear device, andhthent p toi the.

picture Yreceiving;apparatusS. 1

' It Will-be inotedi` that 'each' yf' the'systemsi heretofore disclosed confers advantages. with: respect.to-sensitivityfto.variationsfini attenuation from time to ltime onstheflinefbut 'at the eXpense-ofsomefother factor. In `fthe case i of the poWercurve-s such as illustratedbyt-B in Figi l; fthe. sensitivi-ty .tojnoise and i echoes. is.

increased. 0n account ofthe'wider'rangein value :offline .current required: 'for transmis--v Lio sion,f'the sameis.trueoflthesystem .represented by the. curiveD-vr fori. both Bi and. l) nthefline. current rangesfrom la to .f-,i as' shown onthe scale of ordinates on'Fig. 1. .Curve C does not have this disadvantage but does have the disadvantage, like curve D, of introducing a distortion in the contrasts of the picture in thecase ofall transmission changes except those that are comparatively small. All lof these systems require a more nearly distor! tionless transmission line. rIhe frequency range is increased considerably and the necessity for nearly uniform delay in transmission ofthe various components at different frequencies is increased accordingly. 'I-Ioivever, as shown in Fig. 10, the uniform delay can be secured, if necessary, by theintroduction of a phase equalizer. `IVe claim v VVV1. Inthe transmission of picturesby electric currents, the method Which consists lin sending currents varied in a non-linear relation to the lightY values of the successive elements of the picture to be transmitted, and at the receiving` end exposing corresponding elements of a sensitivesurface to light varied in inverse non-linear relation to the received current. i

2. In the transmission of pictures by elec- Vtric currents, the method which consists in sending currents dependent on thek light values .of successive elements -of the picture to betransmitted,'continuously varying said currents 1n a manner advantageous for transmission, at the receiving end exposing corresponding elements of a sensitive surface to lightdependent in value on said currents as received-and continuously varying saidlight ina manner advantageousfor recording the picture in proper degree of shade. Y

3; In the transmission of pictures by electric currents, the method. which consists in 'sending currents Whose rate of increase with picture density is greater atv large values of picture density than small values, and at the receiving end vexposing ay sensitive surface to light variedin inverse relation to the said currents..` .l

4. In combination in an electrical system 4for.transmitting images of pictures or other Yobjects, means at the sendingend to vary the transmittedk current as a non-linear function of the light valuesof the successive elements ofthe picture or other object, a line to carry the said current to the receiving end, and means atthe receiving end to varythe exposure cfa receiving element in yinverse relation to the received current.

5. In combination in an` electrical system for transmitting images of pictures or other objects, means at the transmitting end to vary the transmitted current as a function of light values of successive elements of the picture or other object-.Whose image is to be transmitted, and Vmeans at the receiving end to compensate the current for `phase distor-i' tion in transmission and .to expose correrspending elements of a receiving medium to light varied as a function ofthe compensated current.

`6. The method lof electro-optical transmission which consists in producing` electrical currents Whose instantaneous values vary in accordance with the licjht tone values of successive elcmentsof a picture or otherv object Whose image is to be transmitted, employing said currents in producing electrical current Whose instantaneous values are a non-linear function of the light tone values of successive elements ofthe picture or other object Whose image isto be transmitted. and sending corresponding non-linear currents over a line and applying them at the receiving endto control the degree of exposure of corresponding successive elements of a receiving medium.

v7. The method of transmitting electric currents and thereby producing images of pictures or other objects Which comprises producing electrical currents varying in accordance with thelight tone values of successive elements of a picture or kother objects to be transmitted,employing these currents to produce currents having a non-linear function of said light tone values, transmitting them to the receiving end and applying them to control the exposure of corresponding' elements of a picture or image receiving medium.

8. The method of transmitting electric current and thereby producing images of pictures lor other objects Which comprises generating an electric current corresponding to the light toneV values of the successive elements of a picture or other object Whose image is to be transmitted, sending this current over a line, compensating the received current for phase distortion, and applying the compensated currentto control the production of an image of said picture or other object.

9. The method of picture transmission which consistsV in varying an electric current asa function of picture density, causing the current to vary over a Wider range of ratio values than the variation of picture density, transmitting` this current and at the receiving end applying it to control the exposure of a sensitive receiving surface.

10. The method of transmitting electrical current and thereby producing images of pictures or other objects which comprises varying an .electric current as a function of the light tones of the picture or other object, causing` theV current to varyoverav Wider rangev of ratio values than the variation of Ythe light tones of the picture or other obj ect,

IOO

rent-"Whose I,variationsl 'in intensity bear ya linear relation to the light tone values of suc cessive elements of the object, means for using saidphotoelectric' current to produce another photoelectric current. Whose 'variations bear afnonelinear relation to the lighttone values of 'the' elements of4 the object,`a transmission line subject to attenuation changes, means 'for amplifying and-applying said second inenti'oned :photoelectric current to said line, means for receiving lthe transmitting current/,"means for utilizing said received current to produce a light beam Whose variations vin intensity vary inversely and non-linearly as regards the received current,l and means fcr'causing the variations in the intensity of 'the sai'd'light beam to produce an image of the object. y f

12.. The combination 4vof a source of light, means for producing` fluctuations in a beam of light from said source, lightl sensitive cell actuatedby saidl beam thercbyproducing vfluctuating electric current, a second light sensitive cell, a second beam ot light,an apertured battle" plate in-the pathct said second beam, means torcontrolling the current from said second cellv in accordance With the intensity ofthe said luctuatingelectric current 'and in accordance With 1the portion ol' the area of-'f'the yopening in said apertured baille plate exposed to said second beam;

l13.V In an electro-optical 'transmission -system, anpobjectwhose image isl to be transmittedjme'ans for producing a photoelectric current-:Whose instantaneous values bear `a lineanrelation'lto the light tones of `corresponding elements :of said object, means for ntranslating said `photcelectric currentfinto a current Whose instantaneous values vary 'eX- ponen-tially Aas regards corresponding light tone va'lues'o'f the elementsof the object, "a receiving station,means to transmit said second mentioned'` current vtorsaid receiving station, and ineansA at ksaid receiving station for translating the received rcurrent into light variations `which bear a linear relation vto 'said `ligh'ttone values of the elements of saidA Obj'eC-.' j

Y 14. In a system ofv ypicture transmission, a picture itobe transmitted, means for producing currents corresponding in amplitude to the :variations in density at the picture ele,- ments, a transmissionline subjectvto attenuation changes, meansjifor varying the amplitude's of saidfcurrents to compensate ifor said changes prior vto the applicationoi' said currents 'to rsaid line', means for applying said currentsv .to said lines,l a receivingstation, means at'said receiving station to produce a Vbeamof light varying in intensityV according tothe received currents, and` means for insuringthat the variations in intensity' of said beambear alinear relationto thcvariations in 'density of corresponding elements of the picture.

regards the4 variations in the light tone lvalue f of the elements of the picture orobject, a re'- ceiving station, means'for amplifying'and transmitting said pliotoelectr'ic currentsto said receiving station, an velectromagnetic light valve operated by the ampliiied currents, a light receiving medium, anda-flight Y 'baille between said valveand 'said'surface having a light passage' of such. shapejas to compensate for the non-linear "relation -between the amplified 'currents and the light y tone values'of correspondingelements ofthe picture or object.

17. The method ofelectro-opticallyfpro# ducing images by electricy energy transmitted from a distant pointwhich coinprises-fcausing energy at the transmitter representing tone values of the 'objectV tov changewith change of light values of the vvsuccessiveele"- ments of said object at a ,rate diierent-iii one part ofthe range of light values thanatanother, and means at the Vreceiver for compensatingfor the non-rectilinearity of the relaitionship between said rates of changei Y 18. The method ofelectro=optically producing ima-ges, by electric energy transmitted from al distant point Whiclicomprisescausing energy at the transmitter representing tone values of the object tol change With' changel of light values of :the successivevelements'of said object at a rate greater at the small values of light than at the large,.and means at the receiver forv compensating for the nonrectilinearity of the relationshipbetween said rates of change.

19. The method ci? .transmittingelectric current and Athereby producing. imagesfof p-ictures'or. other objects `Which vcomprises sending currentkvaried in a nonelinearrelation tothe light values of the successive `1elefments of the picture or other objectwhose image is to be transmitted,y and at thev receiving end villuminating corresponding y.elements of a receiving medium by'light varied in in-k verse non-linear relation to the received current.' v 20. The method ot'transmitting electric current. and 'thereby producing images 4of vpictures or other objects Whichbcomprises sending current dependent on the light tone values of successive elements oit' the-picture or other object Whose image isto be; transmitted, continuously varyingsaidcurrent in i a manner advantageous 'for transmissiongzat the receiving end exposing corresponding ele- In testimony whereof, We have signed out` ments of ay receiving medium to light depend- `names to this speciiicaticn this 2nd day of ent in Value on said current as received, and June, 1925.

10 continuously Varying said light in ak manner advantageous for producing an image of the ALVA B. CLARK. picture or other object in proper degree of HARRY NYQUIST. shade. Y

DANFORTH K. GANNETT. 

